Industries which produce large amounts of liquid products, such as the brewing, soft drink, dairy and other food-oriented industries, utilize large networks of pipes to transfer liquid from one location to another. For a variety of reasons, it is necessary to periodically switch the pathways of liquid flow through the piping network, and accordingly a large number of switching valves may be included as a part of an overall array where the flow from any one of a number of product supply lines can be routed to a desired product outlet line. For example, a modern brewing facility may have a large number of switching valves which are automatically controlled by a central, programmable control unit.
A primary requirement in the beverage and other food industries is that cleanliness be maintained throughout the facility including, of course, the piping networks. While pipes through which liquid flows continuously are generally self-cleaning, stagnation can, of course, occur at valves which are used only periodically, and achieving cleanliness at valve locations is important.
Because it is necessary that such valves in the closed position positively block any liquid flow between the piping systems, it has become a practice in the industry to use valves having relatively movable valve member parts that provide double seats with a leakage chamber therebetween whereby liquid leaking around either seat is drained externally rather than into the other piping network. In order to clean the leakage chamber between the seats of the closed valve, double-block and vent or bleed valves have been developed which have passageways through which cleaning solutions may be supplied to wash the surfaces of the leakage chamber and exit via the drain.
Although in this manner, it is possible to clean the entire intermediate or leakage chamber within this composite valve member, there is no provision for cleaning the actual seat because the valve member parts will be seated during the anticipated in-place cleaning that takes place. Accordingly, provisions have recently been made in order to separately stroke or lift the individual valve member parts so that a particular line and valve seat can be cleaned while product may be flowing through the outer line. Examples of such valves include those shown in U.S. Pat. No. 4,304,251, issued Dec. 8, 1981, and in British patent specification No. 1,592,732 published July 8, 1981. Improvements in valves of this particular type have been sought.